Hammer Surfacing

Piano hammers are just like they portray…little hammers that beat on the piano strings and make the strings vibrate. They have a little handle called a "hammer shank" (thinner than a pencil) while the hammer itself is made of felt (sheep wool) with a wooden core that the hammer shank fits into. A long line of felt it cut into a special shape then wrapped, glued and sometimes stapled to the wooden core and held with hydraulic pressure until the glue dries. The hammer is designed so that there is a tremendous amount of surface tension at the strike point…the point where the hammer contacts the string(s). This surface tension and compression inside the hammer enables the hammer to bounce off the string with very little contact time with the string.

As a hammer wears, it develops grooves at the strike point. These grooves develop into cuts which not only greatly reduce the surface tension of the hammer, but change the tonal quality of the note. The felt becomes compacted and hard in the bottoms of the string cuts and gives a harsh, uneven tone. Because some keys are used more than others, they will wear differently and provide an inconsistent tone between notes.

Drawing of a hammer that needs surfacing

Drawing of a properly surfaced hammer

If we were to look at worn hammer from the side, you would be able to see the rounded shape in the front where it contacts with the string. Looking from the front, we can see that the cuts in the hammer are flat because the strings are flat. Looking from the side again draw an imaginary line vertically as deep as the grooves in the front of the hammer are. By doing this we can see that the worn hammer slaps the string with a great amount of surface area. Between the large surface area and the increased contact time with the string, many of the harmonics that we consider to be good tone are greatly reduced or even eliminated.

In slow motion videography, we now know that as a hammer moves forward to strike the string, it actually wobbles a considerable amount. Sometimes the strings will contact the deep grooves of a hammer where the felt is compacted. The next time you play the note, it may sound completely different in tone because the strings hit on the fluff between the cuts.

The process of surfacing hammers peels layers of felt off the surface of the hammers down past the bottom of the cuts. This gives back a consistency of tone much like when the piano was new. This can usually be done several times in the life of a piano before it is necessary to replace the hammers.